1. Field of the Invention
This invention relates to a filter arrangement for an X-ray apparatus having an X-ray source for directing X-rays onto a target, and having a filter plate positioned in said X-rays for attenuation of said X-rays before their impingement on the target. More particularly, this invention relates to a filter arrangement for an X-ray apparatus which is determined for radiation therapy and which directs diverging X-rays onto a human body. Still more particularly, this invention relates to a filter arrangement for a linear accelerator.
2. Description of the Prior Art
In many X-ray applications generation of X-rays is required such that the X-rays have an equally local distribution of intensity on a target. In some X-ray applications, however, it is desirable to obtain a non-uniform intensity distribution of the X-ray radiation across the target. Such a non-uniform distribution may have, for instance, an intensity maximum which decreases sharply on one side and which decreases slowly, for instance linearly, on the other side. X-rays having such an oblique local intensity distribution are used, for instance, in radiation therapy. They are applied to certain locations of disease. Deep seats of disease require a high X-ray intensity, whereas higher seats require less intensity to be applied to the body.
In some presently known X-ray apparatus, especially in linear accelerators, so-called wedge filters are used to obtain X-rays having an oblique intensity distribution. These filters are inserted into the radiation path between the X-ray source and the target. To each wedge filter belongs a predetermined energy distribution. According to the wedge angle of the filter plates, different oblique intensity distributions are obtained. In order that the doctor or radiologist can apply the X-ray intensity profile which is well adjusted to the location of the disease under treatment, he must dispose of a plurality of wedge filters having various wedge angles. Therefore, a multitude of wedge filters must be at hand and stored. The purchase of such a multitude of wedge filters can mean a large expense, and there may be difficulties in storing the wedge filters close to the X-ray apparatus. In addition, wedge filters have to be changed when another patient undergoes treatment, which procedure requires some time. Also, only wedge filters having definite, selected wedge angles are available. Wedge angles which may be necessary for irradiation and which lie between the selected wedge angles of the available wedge filters, cannot be used for treatment.